1. Field of the Invention
The present invention relates to a control system for switching between working units and stand-by units, and more particularly, to a system which can realize a change-over constitution between working units and stand-by units in response to the degree of importance of the working unit, especially in a digital multiplex equipment having a plurality of units for multiprocessing signals, such as multiplexing or demultiplexing operations.
2. Description of the Related Arts
In recent semiconductor technology, a large number of circuits are integrated to form, for example, a multiplexing portion or a demultiplexing portion in a multiplex network; these integrated circuits being mounted on a printed circuit board to form a unit. A plurality of these units are contained in one package, in accordance with the number of multiplexing lines in use, to form a multiplexer unit or a demultiplexer unit. Obviously, in such a unit-using structure, a stand-by unit must be provided to improve the reliability of the package, by switching from a working unit to a stand-by unit when a fault occurs in the working unit. In this description, the term "working unit" denotes a unit currently in operation.
In data communication, a multiplexer or a multiplexer unit is provided in which a plurality of data transmission lines are multiplexed to enable the forming of a faster data transmission line, and this multiplexer possesses both multiplexing and demultiplexing functions.
A multiplexer is conventionally used as shown in FIG. 1. For example, data of 1,544 MB/sec is transmitted via four lines to a first multiplexing circuit (e.g., M12), the output of which is multiplexed to transmit data of 6,312 MB/sec to a second multiplexing circuit (e.g., M23) via seven circuits. The seven first multiplexing circuits and the second multiplexing circuit form a multiplexer unit such as M13 MUX. Further, nine multiplexer units are combined therewith to form a third multiplexer unit. Then, twenty-eight lines of data are multiplexed by seven multiplexing circuits M23 to output data of 45 MB/sec, and further, via nine lines for data of 45 MB/sec, multiplexing is carried out via data transmission lines to output data (e g. 405 MB/sec).
In general, the change-over operation or switching operation between working units and stand-by units is carried out as follows.